The present invention relates to an ingot mould roll for continuous casting machines and, more specifically, to the superficial conformation of the counterotating rolls of the ingot mold for the metal strip production.
Usually, strips are obtained by a conventional process that provides for the continuous casting of the flat bloom, followed by an eventual beveling, flat bloom heating and hot-rolling.
This process requires a great energy consumption for the flat blooms heating and for processing of the material. On the other hand, the continuous casting process of strips is a technique that enables to produce slim strips, directly as a cast product, thus eliminating the beveling and hot-rolling processes and maintaining the cold-rolling for the slimmer thicknesses only.
Therefore, in the case of a continuous casting of the strips the conditioning of their surfaces is clearly impossible, and it is therefore absolutely necessary that the strip be produced with the all but total absence of surface flaws. To obtain this result the solidification of the strip close to the ingot mold roll has to be piloted.
To solve this problem different solutions are available in art. One solution is based on the embodiment of predetermined gaps on the ingot mold surface. For instance, in the european patent application EP O 493 290 A1, a mobile wall for the metal solidification is described wherein a suitable wrinkledness is embodied on a copper chaplet, and whose cavities thereof are filled with a lower conductivity metal. Thus, a surface is created wherein an alternance is present between higher and lower thermal exchange zones.
Moreover, in the european patent EP O 309 247 B1 that result is obtained by making a series of cavities, of circular or oval shape, and with a regular disposition on the surface of each ingot mold roll.
In both cases a strip is produced with a wrinkled surface thereof. In the applications wherein the surface aspect is determinant this fact causes problems, as even the subsequent processes may be inadequate to restore the original wrinkledness. Moreover, the aforementioned solutions do not solve the problem on those roll surface zones that are left unaffected by the process, and therefore wherein very intense thermal exchanges occur.
Finally, in the first case the necessity of filling the cavities with a second metal and, in the second case, the necessity of emboding regularly spaced cavities on the roll surface, entails a great complexity and therefore a high cost of the embodiment and of the subsequent ingot mold maintainance.
Therefore, the present invention aims at solving the aforementioned drawbacks and at providing an ingot mold wherein the heat extraction and the metal solidification are functions of the ingot mold design, rather than of the inherent features of the material.
Therefore, the invention provides a cooling roll having surface dimples, for continuous casting machines with a pair of counterotating rolls in contact with the molten metal, characterized in that the surface dimples thereof consist in a multiplicity of cavities of a first and a second dimensional order, unevenly spaced on its surface, and optionally, being at least partially in contact among them.
According to the invention, the first order of cavities serves to the purpose of reducing the thermal flow between the solidifying metal and the roll. The depth of these cavities may vary between 2 and 10 xcexcm, and their equivalent diameter is comprised between 10 and 50 xcexcm.
Always according to the invention, the second order of cavities serves to the purpose of creating local gaps. Thus, the solidification is piloted by making it discrete rather than continuous, thereby breaking up and reducing the stresss caused by the shrinking that cause the strip flawing. By virtue of the presence of the first order of cavities that performs a first reduction of the thermal flux, this second order is constituted of cavities of different shapes and of dimensions contained in an equivalent diameter comprised between 0.2 and 1 mm, while the depth of each cavity thereof is comprised between 40 and 200 xcexcm, with a consequent advantage for the surface aspect of the strip.
The fact that the cavities are randomly spaced on the roll""s surface helps in the breaking and reducing of stresses due to the shrinkage, avoiding any preferential direction in which single stresses can combine locally, rising higher values able to cause micro-macro defects.
Moreover, the distance between each of the first order cavities is comprised from 0 to 60 xcexcm, while the distance between each of the second order cavities is comprised from 0 to 1.5 mm.
Furthermore, the pattern impressed on the roll surface of each cavity of at least one of said first and second orders of cavities has a shape selected from the group comprising the polygonal and the circular shape and, more preferably, rhomboidal.
According to the invention, the surface of each roll that comes in contact with the cast metal is constituted of a material having a thermal conductivity comprised between 15 and 380 W/mxc2x7K. and is embodied in a material selected from the following group: steel, copper, nickel and chrome, and/or their alloys, selected in order to provide the constancy of cavities for the entire rolls life.
Advantageously, according to the present invention, with an appropriate webbing of the ingot mold surface, the system is capable of producing flawless strips possessing a controlled roughness such that it can be restored in the subsequent processes.
Specifically, according to the present invention, the webbing is obtained with a peening process of the surface of the rolls, in order to produce cavities on the same roll surface.
Advantageously, according to the present invention, cavities are closely distributed, randomly spaced, of shape that may vary according to the kind of shots employed and anyhow characterized in that it comprises the presence of two different orders of cavity. Thus, the solidification is piloted by making it happen in a discrete rather than continuous way, thereby breaking up and reducing the stresses due to the shrinking that cause the strip flawing.